2 Answers
2

In the Media Blitz mission, in the lower right area of the map (by the mineral fields), you will see a semi-hidden path. Go to the end and destroy the Science Facility building at the end of this path. You should pick up some secret documents that will give you the secret mission. This mission must be played while you're still on the Hyperion.

Does the game allow you to go back after you complete the campaign and pick is up?
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DaveKAug 15 '10 at 11:47

2

@DaveK You can unlock the mission after finishing the campaign, but there is no way to play it at that point. Sadly you need to start a new campaign (or load an old save) in order to play the secret mission.
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AdeeseAug 15 '10 at 12:01